Diffusers are devices used to slow the velocity of a fluid flow by directing the fluid through a flow path of increasing cross-sectional area in the direction of the flow. As the flow area expands and the flow velocity decreases, the dynamic head of the fluid decreases and the static head of the fluid increases.
In a combined cycle power plant, the hot exhaust gas from a gas turbine engine is directed into a heat recovery steam generator (HRSG) in order to transfer heat from the hot gas, thereby cooling the gas before it is exhausted into the atmosphere. The recovered heat warms water passing through tubes of the HRSG and produces steam, which is then used to drive a steam turbine. It is known to install a diffuser between the exit of the gas turbine and the entrance of the HRSG in order to protect the tubes from excessively high velocity gas and to improve the heat transfer performance of the HRSG. U.S. Pat. No. 7,272,930 describes one such combined cycle power plant diffuser application.
A typical diffuser used upstream of a HRSG in a combined cycle power plant includes an outer wall having a generally conical shape which expands in diameter in the downstream direction. Two parameters are used to describe such a diffuser: the area expansion ratio (outlet cross-sectional area divided by inlet cross-sectional area) and the expansion angle (or half-angle, expressed as the angle defined between one side of the wall and a flow direction centerline as viewed in cross-section). These two parameters control the overall length of the diffuser necessary to obtain a desired degree of flow slowing. If the expansion angle is too small, the diffuser is excessively long, which is undesirable in a power plant for space and cost reasons. If the expansion angle is too large, the flow separates from the wall and generates a reverse flow region along the wall, thereby reducing the functionality of the diffuser. The separated flow is unsteady and the separation bubble can move around in the diffuser, adversely affecting the downstream HRSG. Thus, diffusers for combined cycle power plants are generally designed to be conservatively long in order to avoid flow separation over an entire range of power plant operating parameters.
Studies have shown that it is possible to actively control flow separation in a diffuser by exciting vortex interactions in the separated shear layer, such as with acoustic energy, resulting in a reduction of the reattachment length. An active solution for a combined cycle power plant application is difficult because the shear layer can move within the diffuser, and because acoustic excitation requires knowledge of the optimal forcing frequency and amplitude in order to avoid potentially causing the reattachment length to grow. Active solutions also have the disadvantage of consuming power, and the imposed energy may have an adverse impact on the mechanical components of the system.
Studies have also shown that flow trip tabs can reduce flow separation reattachment length of a shear layer by generating longitudinal vortex pairs which increase mixing. A flow tab solution for a combined cycle power plant application is also difficult due to the uncertain location of the shear layer, and such tabs would create a relatively high energy loss due to the abrupt flow disturbances caused by the tabs.